This invention relates generally to fluid-actuated machines for performing a variety of punching operations such as bending, cutting, perforating, and wrenching, and more particularly to portable, one-hand-operable, hydraulic punching machines comprising a hydraulic cylinder driven by one or more pumps for any desired punching operation. Still more particularly, the present invention pertains to improvements in or relating to a return valve which is held closed during the extension of the hydraulic cylinder and which opens to permit the hydraulic fluid to return from the cylinder fluid chamber to the pump fluid chamber during cylinder contraction.
The portable, hydraulic punching machine of the general character set forth above is described and claimed in Japanese Patent Publication No. 6-75738. The return valve normally holds the fluid return passageway open under spring pressure and closes the same in response to fluid pressure from a pump that is driven concurrently with the other pump or pumps for powering the cylinder. The punching machine is operable merely by a finger actuation of a trigger switch for on-off control of an electric motor for driving the pumps.
Typically, the return valve has a spring-loaded spool reciprocably received in a spool chamber which is open to the fluid return passageway and which also communicates with one of the pumps thereby to be actuated against the bias of the spring. A clearance is provided around the spool in order to permit excess fluid to escape from the spool chamber back into the pump fluid chamber.
Difficulties have been encountered in determining the clearance of the return valve spool because the viscosity of oil or like hydraulic fluid is subject to change with its temperature: the higher the temperature, the lower the viscosity. Conventionally, the clearance was made as small as 0.1 millimeter or so at a maximum in order to assure proper oil leakage from the spool chamber when the oil rose in temperature, and so dropped in viscosity, after a prolonged period of use of the machine; otherwise, the heated oil would have leaked so fast that the spool would have failed to close the return passageway in opposition to the force of the spring.
The noted setting of the spool clearance is unsatisfactory, however, when the oil temperature is very low, as immediately after the machine is set into operation, especially in cold weather. The oil viscosity is then so high that, conventionally, leakage through the small spool clearance tended to drop to less than the required rate. Excess pressure often built up in the spool chamber, overloading the drive motor even when the machine itself was not in use and, in the worst case, ruining the piston seals of the pump. The machines of this type were in need of frequent repairs for these reasons, and their useful life was severely limited.
Making the spool clearance greater would provide no remedy at all. It would serve only to cause excessive oil leakage at lower oil temperatures than heretofore. It is apparent that this problem cannot be overcome merely through adjustment of the spool clearance, inasmuch as hydraulic machine oils as we have them today are unavoidably susceptible to wide change in viscosity with temperatures.